1. FIELD OF THE INVENTION
This invention relates to a method for the electroless plating of a plastic substrate and more particularly, it is concerned with a method of forming a metallic thin film excellent in adhesiveness and surface smoothness on a plastic substrate by electroless plating.
2. DESCRIPTION OF THE PRIOR ART
The electroless plating technique has widely been employed as a method of forming a metallic thin film on the surface of an electrically non-conductive substrate. The electroless plating technique means a chemical reduction plating method wherein a metal ion to be plated is reduced with a reducing agent in a plating solution and deposited on a substrate under metallic state. In order to start and accelerate the above described reducing reaction on the surface of a substrate, it has been carried out to subject the substrate to a pretreatment whereby a metal such as palladium, gold or platinum is adsorbed on the surface to give a catalytic activity thereto. This method can be applied to various substrates as is well known, but, for the purpose of increasing the adhesiveness of a plating, in general, it has been proposed to roughen the surface of a substrate chemically or mechanically to thus increase the anchor effect. For example, the surface roughening is carried out by immersing a substrate in a solution of strong acid or strong alkali such as a mixed solution of a bichromate and sulfuric acid or sodium hydroxide, or by subjecting a substrate to mechanical polishing such as blasting or tumbling.
Of late, various studies have been made on a method of producing a magnetic recording medium comprising forming a ferromagnetic metal thin film as a magnetic recording layer by electroless plating or by electroplating further on an electroless plating. One of advantages obtained by forming a ferromagnetic metal thin film as a magnetic recording layer of a magnetic recording medium is that it is possible to use a magnetic recording layer with a thin thickness due to a large saturated magnetic flux density and to obtain a relatively high coercive force suitable for a high density recording. Another advantage obtained by the use of a thin film of ferromagnetic metal is to obtain readily the film with a thin and uniform thickness by electroplating, electroless plating, etc. In the case of providing a thin film of ferromagnetic metal as a magnetic recording layer, however, the surface smoothness of the thin film is directly affected by that of a substrate due to the feature of being thin, so the above described method of increasing the adhesiveness through the anchor effect is not desirable, which results in difficulties to obtain a metallic thin film excellent in surface smoothness.
In the electroless plating method of the prior art, furthermore, it has been thought necessary to render the surface of a substrate sufficiently hydrophillic and then immerse in a pretreatment solution for activation. The quantity of a noble metal such as palladium, gold or platinum adhered in excess and carried away by the substrate surface rendered hydrophilic in the pretreatment for activation amounts to 100 times or more as much as that required actually for the activation of the surface. The palladium, gold or platinum adhered in excess to the substrate is thus brought into an electroless plating bath, resulting in acceleration of the decomposition of the plating solution.
In the above described prior art method, not only it is difficult to obtain a plating with a good surface smoothness, but also a noble metal such as palladium necessary for the activation of the surface of a substrate is wasted and tends to lower the stability of a plating bath. Therefore, the prior art method is disadvantageous in that a high technique is required for controlling the processing bath which stability is inferior.
For the purpose of forming a magnetic layer, furthermore, various studies or proposals have hitherto been made on the adhesiveness, surface smoothness and processing stability of a metallic thin film prepared by plating. For example, a method of providing an active surface for chemical plating by forming a metallic film through vapor deposition in vacuo or the like, or a method of providing an electrically conductive surface for electroplating is known as disclosed in Japanese Patent Publication No. 9490/1969, Japanese Patent Application (OPI) Nos. 97876/1974 and 6308/1975, U.S. Pat. No. 3,801,368, etc. Another proposal for the activation of the surface of a substrate is, for example, disclosed in Japanese Patent Publication No. 22983/1967 (U.S. Pat. No. 3,525,635) in which the activation is carried out by forming a thin continuous undercoated layer consisting of an insulating polymer in which colloidal metallic particles are uniformly dispersed capable of giving a nucleus forming surface for chemical plating. The similar methods are disclosed in Japanese Patent Publication Nos. 29242/1968, 126679/1970 and 37646/1970, Japanese Patent Application (OPI) Nos. 78038/1973, 53537/1974, 63623/1974 and 73337/1974 and U.S. Pat. No. 3,594,229.
The above described method for activating the surface of a substrate by undercoating need not utilize the anchor effect unlike the prior art activation method by chemical processing using stannous chloride or palladium chloride. Consequently, a metallic thin film apparently excellent in surface smoothness can be obtained with a good stability of processing because a noble metal adhered in excess is hardly brought into a chemical plating bath. In the above described method for activating the surface of a substrate by undercoating, however, the undercoating is generally carried out by dispersing a noble metal such as gold, silver, platinum or palladium in a polymer and a portion in which the noble metal can be utilized as nuclei for chemical plating is thus limited to only a surface portion of the undercoated layer, resulting in increase of the cost per unit area. Since water or hydrogen chloride is often added to an undercoating solution of a high molecular material and organic solvent for the purpose of dissolving a noble metal salt, the adhesion of the undercoated layer to a substrate is not always strong. Furthermore, such a difficulty in dispersing sufficiently a noble metal in a high molecular material results often in pinholes in a metallic thin film deposited by chemical plating. Another proposal for the activation of the surface of a substrate is disclosed in Japanese Patent Publication No. 7843/1968 in which the activation is carried out by processing a surface to be plated with a catalyst consisting of a solution of a palladium chloride complex with water and hydrogen halide dissolved in an organic solvent before immersing in a plating bath. In the above described method for processing a substrate to be plated with a mixed solution of water and an organic solvent containing palladium chloride, the substrate to be surface activated is much limited and the bonding strength of the palladium chloride complex to the substrate is unstable due to hydration so that a metallic thin film deposited by chemical plating has often a poor adhesiveness.
Methods comprising providing an undercoated layer on a substrate and effecting plating on the undercoated layer are disclosed in Japanese Patent Publication Nos. 19467/1964, 10239/1970, 4071/1971, 21921/1971, 33161/1973 and 34161/1973 and Patent Application (OPI) Nos. 10530/1972 and 57925/1975. These methods, however, aim at improving the adhesiveness at the expense of the surface smoothness by providing an undercoated layer and etching for the surface roughening or by dispersing metallic fine particles in an undercoated layer and, therefore, cannot be applied to the production of a magnetic recording medium using a magnetic plating layer as a recording layer.